Flat panel displays, e.g., liquid crystal displays, are known. Recently, the use of plasma channels to address a liquid crystal display (LCD) has become known. For example, U.S. Pat. Nos. 4,896,149, 5,036,317, 5,077,553, 5,272,472, 5,313,223, the disclosures of which are all hereby incorporated by reference, each disclose such a structure. This type of display technology provides an active addressing matrix suitable for high-line-count displays, and is competitive alternative to the known thin-film transistor (TFT) active matrix approach.
These plasma channel panels are also referred to herein as plasma addressed liquid crystal (PALC) displays. This type of plasma display panel is generally formed of two parallel substrates separated from each other to form a discharge space between the substrates, which contains a discharge gas, such as a mixture of helium, neon and xenon. The inner-facing surface of each of the substrates bears a pattern of spaced parallel electrodes, with the electrodes on one substrate being oriented, for example, in a direction orthogonal to the direction of the electrodes on the other substrate. The electrode bearing surfaces of the substrates are typically covered with a dielectric layer, and red, green and blue phosphors are separately located in discrete areas on the internal surface of the dielectric layer on one of the two substrates. The dielectric layers are generally lead-based glass frits fired between 500 and 600.degree. C., depending on their formulation and the level of uniformity required. The displayed picture is produced by plasma discharges which are induced locally in the gas by applying a suitable voltage between the electrodes of one substrate and the electrodes of the other substrate. Ultraviolet light emitted locally by the gas discharge induces luminescence of the neighboring phosphors.
A PALC display relies on the highly non-linear electrical behavior of a relatively low pressure (e.g., 10 to 100 Torr) gas, e.g., He, confined in parallel channels. A cross section of a portion of a PALC display 100 is shown in FIG. 1. A pair of parallel electrodes 101A (anode) and 101C (cathode) is deposited in each channel 102 on a rear glass plate 101G, for example, forming the bottom of the channels, and a very thin dielectric sheet 103, e.g., a glass micro-sheet of about 50 .mu.m thickness, forms the top of the channels 102. A liquid crystal layer 104 on top of the micro-sheet 103 is the optically active portion of the display 100. A cover sheet 105, e.g., a passive glass plate of about 1.1 mm, with transparent conducting electrodes, e.g., made from indium-tin oxide (ITO), running perpendicular to the plasma channels 102, lies on top of the liquid crystal 104. Conventional polarizers 106, color filters 107, and back lights 108, like those found in other conventional liquid crystal displays, are also commonly used, as illustrated.
When voltages are applied to the transparent electrodes, since there is no ground plane, the voltages are divided among the liquid crystal, the micro-sheet, the plasma channel, and any other insulators intervening between the transparent electrode and whatever becomes the virtual ground. As a practical matter, this means that if there is no plasma in the plasma channel, the voltage drop across the liquid crystal will be negligible, and the pixels defined by the crossings of the transparent electrodes and the plasma channels will not switch. If, however, a voltage difference sufficient to ionize the gas is first applied between the pair of electrodes in a plasma channel, a plasma forms in the plasma channel so that it becomes conducting, and constitutes a ground plane. As a result, for pixels atop this channel, the voltages will be divided between the liquid crystal and the micro-sheet only. This places a substantial voltage across the liquid crystal and causes the pixel to switch. Igniting a plasma in the channel causes the row above the channel to be selected. Because the gas in the channels is non-conducting until a well-defined threshold voltage between the electrode pair is reached, the rows are extremely well isolated from the column voltages unless selected. This high non-linearity allows large numbers of rows to be addressed without loss of contrast.
To avoid luminous cross-talk between neighboring regions and improve the contrast in such displays, opaque barrier ribs 110 are disposed on at least one of the substrates (typically the rear one) forming electrically insulated discharge cells. The barrier rib structure is typically periodic with a pitch of, for example, from 200 .mu.m to 400 .mu.m, depending on the panel resolution. These ribs are, for example, about 30-100 .mu.m wide and 100-200 .mu.m thick (i.e., high).
Alternatively, a closed cell design has been employed having square cells which are about 200-400 .mu.m on each side. The "ribs" which form these square cells are about 30 .mu.m to 70 .mu.m wide and about 30 to 200 .mu.m high. Plasma panels of this type are described, for example, in U.S. Pat. No. 4,853,590, as well as Japanese Patent Application Nos. J04255638 and J04075232. The networks of parallel barrier ribs mentioned above delimit columns of pixels which can be addressed independently. The two perpendicular networks of electrodes allow ionization of the gas at the selected pixels. The ultraviolet radiation emitted by the ionized gas causes the excitation of areas of phosphorescent products associated with said pixels according to the configuration of an image which is to be displayed.
The PALC display relies on the use of a thin micro-sheet to separate the plasma from the liquid crystal. This micro-sheet should be as thin as possible (e.g. 1.5-2 mils), with as high a dielectric constant as possible, to thereby minimize the voltage drop across it. Current display manufacturers utilize a single, monolithic piece of micro-sheet for this purpose, e.g., a D-263 micro-sheet of 30 to 50 .mu.m thickness made by Schott. However, these large sheets of glass are difficult to manufacture, causing the availability of large, thin micro-sheet to be a potential limitation on the size of the PALC displays that can be made.
In the past, the barrier ribs have typically been made either by a silk-screening method, or by sandblasting from a deposited layer of frit. Thus, the channels between the barrier ribs have been made by etching into a glass substrate or by building up walls of glass on a substrate by deposition processes such as screen-printing. However, etching of the channels typically results in channels having rounded bottoms, while building up material to form walls generally results in non-vertical side walls. Both of these conditions adversely affect light transmission through the panel. In addition, the manufacture of rib structures with a high aspect ratio usually requires multiple process steps, including polishing the top of the ribs to match the flatness of the glass micro-sheet.
Accordingly, a need exists for a method which solves the above problems and overcomes the limitations of the known manufacturing processes of PALC displays. There is further a need for a method which achieves improved structures on the rear glass plate, including metallic electrodes and high aspect ratio opaque ribs, and which can obtain such a structure with a thin dielectric glass sheet.